1. Field of the Invention
This invention relates to a skid control system for a vehicle having two or more wheels and the respective brakes for the wheels, and more particularly to a skid control system which includes two or more skid control circuits, each of which includes a wheel speed sensor for providing an output signal representative of the speed of the wheel or the mean speed of the wheels, an approximate vehicle speed generator for providing an output signal representative of the approximate speed of the vehicle on the basis of the output signal of the wheel speed sensor, a slip signal generator receiving the output signal of the wheel speed sensor, an acceleration/deceleration signal generator receiving the output signal of the wheel speed sensor, a valve control circuit receiving the outputs of the slip signal generator and acceleration/deceleration signal generator, and electromagnetic inlet and outlet valves receiving the output of the valve control circuit, wherein the maximum of the outputs of the approximate vehicle speed generators of the skid control circuits is supplied to the respective slip signal generator, a predetermined amount is set in the respective slip signal generators, and a slip signal for relieving the brake to the wheel is generated from the slip signal generator, when the speed of the corresponding wheel becomes lower by more than the predetermined amount than the maximum output of the approximate vehicle speed generators.
2. Description of the Prior Art
In a conventional skid control system for a vehicle having plural wheels and the respective brakes for the wheels, a wheel speed sensor is associated with the wheel or wheels for detecting the speed of the wheel or the mean speed of the wheels. Approximate vehicle speeds estimating the speed of the vehicle are produced from the detecting outputs of the wheel speed sensors, respectively. The maximum of the approximate vehicle speeds is selected as a common approximate vehicle speed to the wheels. Or an approximate vehicle speed is produced from the maximum of the detecting outputs of the wheel speed sensors. The wheel speeds are compared with thus produced approximate vehicle speed in slip signal generators, respectively. When the wheel speed becomes lower by more than a predetermined amount than the approximate vehicle speed, or when slip of the wheel speed to the approximate vehicle speed becomes larger than a predetermined value, the slip signal generator generates a slip signal. Acceleration of the wheel and deceleration thereof are produced from the detecting output of the wheel speed sensor. When they become larger than predetermined thresholds, an acceleration signal and a deceleration signal are generated from an acceleration/deceleration signal generator. A control signal for controlling the brake for the wheel is obtained from a valve control circuit on the basis of the slip signal, acceleration signal or deceleration signal.
In the above-described skid control system, the wheel speed is used as the approximate vehicle speed, until the deceleration of the wheel reaches a predetermined level. After that, such a signal that decreases at a predetermined gradient with time, is used as the approximate vehicle speed. The wheel speed at the time when the deceleration of the wheel has reached the predetermined level is equal to the approximate vehicle speed.
When the deceleration of the wheel reaches the predetermined level, a considerable brake is already applied to the wheel. Accordingly, some slip occurs in the wheel. For example, slip of a few or several percentages occurs in the wheel. The approximate vehicle speed is lower by slip of a few or several percentages than the true vehicle seed.
The approximate vehicle speed is produced from the plural wheel speeds. Accordingly, when all of the wheels are uniformly or considerably braked to slip of a few or several percentages, the produced approximate vehicle speed is lower by the value corresponding to the slip than the true vehicle speed. Or when any one of the wheels is not so considerably braked, and the slip of the corresponding wheel is nearly equal to zero, the approximate vehicle speed is nearly equal to the true vehicle speed.
However, in the conventional skid control system, the difference between the approximate vehicle speed and the true vehicle speed is neglected, and the predetermined amount is set in the slip signal generator. When the wheel speed becomes lower by the predetermined amount than the approximate vehicle speed, the slip signal is generated from the slip signal generator. When the predetermined amount in the slip signal generator is determined under the assumption that all of the wheels are uniformly or satisfactorily braked, it is too small for the wheel which is not satisfactorily braked. Before the corresponding wheel is satisfactorily braked, the slip signal is generated from the slip signal generator, and the brake for the corresponding wheel is relieved. On the other hand, when the predetermined amount in the slip signal generator is determined under the assumption that any one of the wheels is not satisfactorily braked, it is too large for the case that all of the wheels are uniformly or satisfactorily braked. As the result, all of the wheels slip too much with respect to the optimum slip.